If I want to affect a creature with a concentration spell and it succeeds its save, do I notice my concentration failing? [duplicate]

I’m a fairly new DM, and I’ve run into a problem. One of my players tried to cast Modify Memory on a golem. Golems are immune to being charmed, but Modify Memory doesn’t say creatures immune to charm are unaffected. I decided to let the spell through because of narrative reasons. But, we had a discussion.

Modify Memory says:

On a failed save, the target becomes Charmed by you for the Duration.

The spell is a concentration spell for 1 minute. My question is the following: If I had not let the spell circumvent the golem’s immunity to charm, would the caster know that the spell didn’t affect the target, since it requires concentration, but there’s nothing to concentrate on?